Farmers need to watch for drought-induced herbicide carryover

October 24, 2012

WEST LAFAYETTE, Ind. - This summer's lack of rain has
translated into the potential for summer-applied herbicides to carry over into
winter wheat or even spring-planted corn and soybean crops - something growers
need to be monitoring, two Purdue Extension weed scientists say.

In a normal year, soil moisture helps dissipate herbicide
soil concentrations, but in a drought year, the lack of moisture slows that
process. Some herbicides might carry over and exceed the labeled crop rotation
restrictions meant to prevent injury to the next crop.

"The largest
concern this year is the carryover of atrazine and subsequent injury on
wheat," Bill Johnson said. "It is off-label to plant any crop other
than corn or sorghum during the same calendar year of an atrazine
application."

Another herbicide
with potential to injure wheat is fomesafen applied postemergence in soybeans.
The wheat rotational restriction for fomesafen is four months after
application, but in areas that saw the least rainfall, Johnson said the
carryover could be longer.

"Producers
who applied a fomesafen product to soybeans this summer and have not seen
significant rain following application should be aware of the potential for
injury on emerging wheat," he said.

The return of rain
to some areas has reduced some of the concern for herbicide carryover into
spring-planted crops, but it hasn't eliminated it altogether.

According to
Travis Legleiter, Purdue Extension weed scientist, producers in areas that
haven't had significant rain should still be aware of the potential for
atrazine and HPPD inhibitors to carryover into soybeans - especially in high pH
or high clay content soils.

He also suggested
that producers be wary of potential imidazolinone chemistry carryover into
spring-planted corn.

Growers concerned
about herbicide carryover have two options for analyzing soil. The first is to
conduct a bioassay, a method of planting susceptible crop seeds into suspected
soil and comparing the growth and injury to plants grown in a non-herbicide
treated soil. A bioassay can be done in the field or in containers.

The second option
is to take soil samples from suspected carryover fields and have them analyzed
by a commercial lab, which can be costly.

"Both
bioassays and lab analysis should either be done in late fall or early spring
to allow for maximum herbicide degradation and provide a more representative
result of potential injury at planting," Legleiter said.